1. Field of the Invention
This invention describes a single coil that can be used to measure simultaneously two components of the magnetic field or two components of the gradient of the field; in particular, we describe a coil, primarily intended for measurement of one component of the gradient, having several attractive properties and that is easily modified to measure two independent gradient components simultaneously.
Weak magnetic fields are often measured by placing a pickup coil to the field to be measured, and connecting it to a current sensing element. When ultimate sensitivity is required, the coil is most often made of superconducting material, and it is connected to a superconducting quantum interference device (SQUID). Such an instrument has been described in detail, for example, in an article appeared in Journal of Low Temperature Physics, vol. 76 (1989), issue 5/6, pp. 287-386. The pickup coil can be either coupled to a signal coil laid on top of the SQUID, thus coupling the signal magnetically to the SQUID, or the pickup coil can be a galvanically coupled part of the SQUID loop. It has been shown that in order to reach the best sensitivity, the inductance of the SQUID has to be very small, only of the order of 10.sup.-11 H, and that stray capacitances across the loop have to be minimized.
An optimal pickup coil has, in general, an equal impedance to that of the input of the measuring element. When a SQUID is used to measure the current flowing in the pickup coil, it is advisable to use a pickup coil with a small inductance. Then, the signal coil laid on top of the SQUID can have only a small number of turns and thus a small parasitic capacitance across the SQUID loop; this means better sensitivity. This follows from the fact that the inductance of the signal coil, that is the input inductance of the current sensing element, is directly proportional to the product of the square of the number of turns in the signal coil and the inductance of the SQUID loop. The requirement that the inductance of the SQUID loop should be as small as possible makes it difficult to use the pickup coil as a galvanically coupled part of the SQUID.
2. Description of the Related Art
Ultrasensitive magnetic field detectors are needed, for example, when the extremely weak magnetic signals generated by the human brain are measured. In medical research and diagnostics this method is gaining more and more attention since with it it is possible to locate the source currents associated with the brain activities with a spatial and temporal resolution of a few millimeters and milliseconds. The measurement must be performed simultaneously at several locations; even the measurement of over one hundred magnetic signals all over the skull is necessary. It has been shown that it is advantageous to measure, instead of the magnetic field itself, certain components of the gradient of the field; especially this applies to the planar gradients .differential.B.sub.z /.differential.x and .differential.B.sub.z /.differential.y (see, for example, SQUID'85: Superconducting Quantum Interference Devices and their Applications, edited by H. D. Hahlbohm and H. Lubbig, Walter de Gruyter, Berlin (1985), pp. 939-944. One such instrument, containing 24 channels, has been described in the book Advances in Biomagnetism, edited by S. J. Williamson, M. Hoke, G. Stroink and M. Kotani, Plenum, New York (1989), pp. 673-676.
FIG. 1 shows two different two different prior-art coil configurations measuring the planar gradient: the parallel (a) and the series (b) configuration. The series configuration has been more popular (see, for example U.S. Pat. No. 4,320,341, EP 210 489, EP 399 499). This mainly because in the parallel configuration, a shielding current is induced in a homogeneous field, tending to cancel the homogeneous field; in the series coil this current is absent. Especially in thin-film pickup coils it is possible that the shielding current exceeds the critical current of the film, and the film becomes non-superconducting. In addition, the shielding currents due to a homogeneous field causes a local inhomogeneous component; in multichannel magnetometers this unwanted gradient is coupled as an error signal to neighbor pickup coils. The parallel configuration has, however, a much smaller inductance, only 1/4 of that of a series coil of the same dimensions. Therefore, parallel connection of loops has been used in devices where the pickup coil is a galvanically coupled part of the actual SQUID loop, as for example in the article appeared in Journal of Applied Physics, vol. 58 (1985 ), no. 11, ss. 4322-4325. Parallel loops have been applied also to reduce the inductance of the SQUID, as in Journal of Applied Physics, volume 42 (1971), issue 11, pp. 4483-4487 and Applied Physics Letters, volume 57 issue 4 (1990), pp. 406-408.
The transverse, planar gradient components must be measured in one single point; thus one must fabricate a structure having two orthogonal gradiometers on top of each other. Such a structure, realized by conventional means, as described in aforementioned publications (U.S. Pat. No. 4,320,341, EP 210 489, SQUID'85: Superconducting Quantum Interference Devices and their Applications, (1985), ss. 939-944) is necessarily relatively complicated.